Method and device for performing access in wireless LAN system

ABSTRACT

One embodiment of the present invention, in a method for enabling a station (STA) to perform an access to a medium in a wireless communication system, is a method for performing an access, comprising the steps of: receiving a frame including an RPS element; checking a restricted access window (RAW) assignment field within the RPS element; and performing an access on the basis of a RAW start time when the STA corresponds to a RAW group related to the RAW assignment field, wherein the RAW start time is obtained on the basis of a start time indication sub field, and the start time indication sub field indicates whether the RAW start time sub field indicating the RAW start time is included in the RAW assignment field.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the National Stage filing under 35 U.S.C. 371 ofInternational Application No. PCT/KR2013/012256, filed on Dec. 27, 2013,which claims the benefit of U.S. Provisional Application No. 61/809,902,filed on Apr. 9, 2013 and 61/845,383, filed on Jul. 12, 2013, thecontents of which are all hereby incorporated by reference herein intheir entirety.

TECHNICAL FIELD

The present disclosure relates to a wireless communication system and,more particularly, to a method and device for performing access in aWireless LAN system.

BACKGROUND ART

With recent development of information communication technologies, avariety of wireless communication technologies have been developed. Fromamong such technologies, WLAN is a technology that enables wirelessInternet access at home, in businesses, or in specific service providingareas using a mobile terminal, such as a personal digital assistant(PDA), a laptop computer, or a portable multimedia player (PMP), basedon radio frequency technology.

In order to overcome limited communication speed, which has been pointedout as a weak point of WLAN, technical standards have recentlyintroduced a system capable of increasing the speed and reliability of anetwork while extending coverage of a wireless network. For example,IEEE 802.11n supports high throughput (HT) with a maximum dataprocessing rate of 540 Mbps. In addition, Multiple Input Multiple Output(MIMO) technology, which employs multiple antennas for both atransmitter and a receiver in order to minimize transmission errors andoptimize data rate, has been introduced.

Machine-to-machine (M2M) communication technology has been discussed asa next generation communication technology. A technical standard tosupport M2M communication in the IEEE 802.11 WLAN system is also underdevelopment as IEEE 802.11ah. In M2M communication, a scenario in whicha small amount of data is occasionally communicated at a low speed in anenvironment having a large number of devices may be considered.

Communication in the WLAN system is performed on a medium shared by alldevices. If the number of devices increases as in M2M communication, achannel access mechanism needs to be efficiently improved in order toreduce unnecessary power consumption and interference.

DISCLOSURE Technical Problem

This specification discloses techniques relating to a method forindicating the start time of a Restricted Access Window (RAW).

Objects of the present invention are not limited to the aforementionedobject, and other objects of the present invention which are notmentioned above will become apparent to those having ordinary skill inthe art upon examination of the following description.

Technical Solution

In a first aspect of the present invention, provided herein is a methodfor performing access to a medium by a station (STA) in a wirelesscommunication system, the method including receiving a frame containinga Restricted Access Window (RAW) Parameter Set (RPS) element, checking aRAW Assignment field in the RPS element, and performing access based ona start time of a RAW when the STA corresponds to a RAW group related tothe RAW Assignment field, wherein the start time of the RAW is obtainedbased on a Start Time Indication subfield, wherein the Start TimeIndication subfield indicates whether a RAW Start Time subfieldindicating the start time of the RAW is included in the RAW Assignmentfield.

In second aspect of the present invention, provided herein is a stationfor performing access to a medium in a wireless communication system,the station including a transceiver module, and a processor, wherein theprocessor is configured to receive a frame containing a RestrictedAccess Window (RAW) Parameter Set (RPS) element, check a RAW Assignmentfield in the RPS element, and perform access based on a start time of aRAW when the STA corresponds to a RAW group related to the RAWAssignment field, wherein the start time of the RAW is obtained based ona Start Time Indication subfield, wherein the Start Time Indicationsubfield indicates whether a RAW Start Time subfield indicating thestart time of the RAW is included in the RAW Assignment field.

When the Start Time Indication subfield is set to 0, the start time ofthe RAW may be determined depending on a position of the RAW Assignmentfield among RAW Assignment fields in the RPS element.

When the RAW Assignment field is a first RAW Assignment field in the RPSelement, the start time of the RAW may be a time immediately aftertransmission of the frame.

When the RAW Assignment field is a second RAW Assignment field or a RAWAssignment field after the second RAW Assignment field in the RPSelement, the RAW start time may be a time immediately after end of aprevious RAW.

When the Start Time Indication subfield is set to 0, the RAW Assignmentfield may not include the RAW Start Time subfield.

When the Start Time Indication subfield is set to 1, the start time ofthe RAW may be determined by the RAW Start Time subfield.

When the Start Time Indication subfield is set to 1, the RAW Assignmentfield may include the RAW Start Time subfield.

The RAW Start Time subfield may indicate a duration from the frame tothe start time of the RAW.

The frame may be either a beacon frame or a short beacon frame.

The performing may include determining a slot for performing accessamong one or more slots included in the RAW, and performingcontention-based access in the determined slot.

The slot for performing access may be determined by an associationidentifier (AID) of the STA.

The RPS element may contain one or more RAW Assignment fields.

Advantageous Effects

According to embodiments of the present invention, even if a beaconframe does not include an RAW Start Time subfiled, an STA is capable ofidentifying the RAW start time. Accordingly, the size of the beaconframe may be reduced.

The effects that can be obtained from the present invention are notlimited to the aforementioned effects, and other effects may be clearlyunderstood by those skilled in the art from the descriptions givenbelow.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are intended to provide a furtherunderstanding of the present invention, illustrate various embodimentsof the present invention and together with the descriptions in thisspecification serve to explain the principle of the invention.

FIG. 1 is a diagram showing an exemplary structure of an IEEE 802.11system to which the present invention is applicable.

FIG. 2 is a diagram showing another exemplary structure of an IEEE802.11 system to which the present invention is applicable.

FIG. 3 is a diagram showing yet another exemplary structure of an IEEE802.11 system to which the present invention is applicable.

FIG. 4 is a diagram showing an exemplary structure of a WLAN system.

FIG. 5 illustrates a link setup process in a WLAN system.

FIG. 6 illustrates a backoff process.

FIG. 7 illustrates a hidden node and an exposed node.

FIG. 8 illustrates RTS and CTS.

FIG. 9 illustrates a power management operation.

FIGS. 10 to 12 illustrate operations of a station (STA) having receiveda TIM in detail.

FIG. 13 illustrates a group-based AID.

FIGS. 14 to 16 illustrate a RAW and an RPS element.

FIGS. 17 to 22 illustrate an embodiment of the present invention.

FIG. 23 is a block diagram illustrating a wireless apparatus accordingto one embodiment of the present invention.

BEST MODE

Hereinafter, exemplary embodiments of the present invention will bedescribed with reference to the accompanying drawings. The detaileddescription, which will be disclosed along with the accompanyingdrawings, is intended to describe exemplary embodiments of the presentinvention and is not intended to describe a unique embodiment throughwhich the present invention can be carried out. The following detaileddescription includes specific details in order to provide a thoroughunderstanding of the present invention. However, it will be apparent tothose skilled in the art that the present invention may be practicedwithout such specific details.

The embodiments of the present invention described hereinbelow arecombinations of elements and features of the present invention. Theelements or features may be considered selective unless otherwisementioned. Each element or feature may be practiced without beingcombined with other elements or features. Further, an embodiment of thepresent invention may be constructed by combining parts of the elementsand/or features. Operation orders described in embodiments of thepresent invention may be rearranged. Some constructions or features ofany one embodiment may be included in another embodiment and may bereplaced with corresponding constructions or features of anotherembodiment.

Specific terms used in the following description are provided to aid inunderstanding of the present invention. These specific terms may bereplaced with other terms within the scope and spirit of the presentinvention.

In some instances, well-known structures and devices are omitted inorder to avoid obscuring the concepts of the present invention and theimportant functions of the structures and devices are shown in blockdiagram form. The same reference numbers will be used throughout thedrawings to refer to the same or like parts.

The embodiments of the present invention can be supported by standarddocuments disclosed for at least one of wireless access systems such asthe institute of electrical and electronics engineers (IEEE) 802, 3rdgeneration partnership project (3GPP), 3GPP long term evolution (3GPPLTE), LTE-advanced (LTE-A), and 3GPP2 systems. For steps or parts ofwhich description is omitted to clarify the technical features of thepresent invention, reference may be made to these documents. Further,all terms as set forth herein can be explained by the standarddocuments.

The following technology can be used in various wireless access systemssuch as systems for code division multiple access (CDMA), frequencydivision multiple access (FDMA), time division multiple access (TDMA),orthogonal frequency division multiple access (OFDMA), single carrierfrequency division multiple access (SC-FDMA), etc. CDMA may beimplemented by radio technology such as universal terrestrial radioaccess (UTRA) or CDMA2000. TDMA may be implemented by radio technologysuch as global system for mobile communications (GSM)/general packetradio service (GPRS)/enhanced data rates for GSM evolution (EDGE). OFDMAmay be implemented by radio technology such as IEEE 802.11 (Wi-Fi), IEEE802.16 (WiMAX), IEEE 802.20, evolved-UTRA (E-UTRA), etc. For clarity,the present disclosure focuses on 3GPP LTE and LTE-A systems. However,the technical features of the present invention are not limited thereto.

Structure of WLAN System

FIG. 1 is a diagram showing an exemplary structure of an IEEE 802.11system to which the present invention is applicable.

The structure of the IEEE 802.11 system may include a plurality ofcomponents. A WLAN which supports transparent station (STA) mobility fora higher layer may be provided by mutual operations of the components. Abasic service set (BSS) may correspond to a basic building block in anIEEE 802.11 LAN. In FIG. 1, two BSSs (BSS1 and BSS2) are present and twoSTAs are included in each of the BSSs (i.e. STA1 and STA2 are includedin BSS1 and STA3 and STA4 are included in BSS2). An ellipse indicatingthe BSS in FIG. 1 may be understood as a coverage area in which STAsincluded in a corresponding BSS maintain communication. This area may bereferred to as a basic service area (BSA). If an STA moves out of theBSA, the STA cannot directly communicate with the other STAs in thecorresponding BSA.

In the IEEE 802.11 LAN, the most basic type of BSS is an independent BSS(IBSS). For example, the IBSS may have a minimum form consisting of onlytwo STAs. The BSS (BSS1 or BSS2) of FIG. 1, which is the simplest formand does not include other components except for the STAs, maycorrespond to a typical example of the IBSS. This configuration ispossible when STAs can directly communicate with each other. Such a typeof LAN may be configured as necessary instead of being prescheduled andis also called an ad-hoc network.

Memberships of an STA in the BSS may be dynamically changed when the STAbecomes an on or off state or the STA enters or leaves a region of theBSS. To become a member of the BSS, the STA may use a synchronizationprocess to join the BSS. To access all services of a BSS infrastructure,the STA should be associated with the BSS. Such association may bedynamically configured and may include use of a distributed systemservice (DSS).

FIG. 2 is a diagram showing another exemplary structure of an IEEE802.11 system to which the present invention is applicable. In FIG. 2,components such as a distribution system (DS), a distribution systemmedium (DSM), and an access point (AP) are added to the structure ofFIG. 1.

A direct STA-to-STA distance in a LAN may be restricted by physical(PHY) performance. In some cases, such restriction of the distance maybe sufficient for communication. However, in other cases, communicationbetween STAs over a long distance may be necessary. The DS may beconfigured to support extended coverage.

The DS refers to a structure in which BSSs are connected to each other.Specifically, a BSS may be configured as a component of an extended formof a network consisting of a plurality of BSSs, instead of independentconfiguration as shown in FIG. 1.

The DS is a logical concept and may be specified by the characteristicof the DSM. In relation to this, a wireless medium (WM) and the DSM arelogically distinguished in IEEE 802.11. Respective logical media areused for different purposes and are used by different components. Indefinition of IEEE 802.11, such media are not restricted to the same ordifferent media. The flexibility of the IEEE 802.11 LAN architecture (DSarchitecture or other network architectures) can be explained in that aplurality of media is logically different. That is, the IEEE 802.11 LANarchitecture can be variously implemented and may be independentlyspecified by a physical characteristic of each implementation.

The DS may support mobile devices by providing seamless integration ofmultiple BSSs and providing logical services necessary for handling anaddress to a destination.

The AP refers to an entity that enables associated STAs to access the DSthrough a WM and that has STA functionality. Data can be moved betweenthe BSS and the DS through the AP. For example, STA2 and STA3 shown inFIG. 2 have STA functionality and provide a function of causingassociated STAs (STA1 and STA4) to access the DS. Moreover, since allAPs correspond basically to STAs, all APs are addressable entities. Anaddress used by an AP for communication on the WM need not necessarilybe identical to an address used by the AP for communication on the DSM.

Data transmitted from one of STAs associated with the AP to an STAaddress of the AP may be always received by an uncontrolled port and maybe processed by an IEEE 802.1X port access entity. If the controlledport is authenticated, transmission data (or frame) may be transmittedto the DS.

FIG. 3 is a diagram showing still another exemplary structure of an IEEE802.11 system to which the present invention is applicable. In additionto the structure of FIG. 2, FIG. 3 conceptually shows an extendedservice set (ESS) for providing wide coverage.

A wireless network having arbitrary size and complexity may be comprisedof a DS and BSSs. In the IEEE 802.11 system, such a type of network isreferred to an ESS network. The ESS may correspond to a set of BSSsconnected to one DS. However, the ESS does not include the DS. The ESSnetwork is characterized in that the ESS network appears as an IBSSnetwork in a logical link control (LLC) layer. STAs included in the ESSmay communicate with each other and mobile STAs are movabletransparently in LLC from one BSS to another BSS (within the same ESS).

In IEEE 802.11, relative physical locations of the BSSs in FIG. 3 arenot assumed and the following forms are all possible. BSSs may partiallyoverlap and this form is generally used to provide continuous coverage.BSSs may not be physically connected and the logical distances betweenBSSs have no limit. BSSs may be located at the same physical positionand this form may be used to provide redundancy. One (or more than one)IBSS or ESS networks may be physically located in the same space as one(or more than one) ESS network. This may correspond to an ESS networkform in the case in which an ad-hoc network operates in a location inwhich an ESS network is present, the case in which IEEE 802.11 networksdifferent organizations physically overlap, or the case in which two ormore different access and security policies are necessary in the samelocation.

FIG. 4 is a diagram showing an exemplary structure of a WLAN system. InFIG. 4, an example of an infrastructure BSS including a DS is shown.

In the example of FIG. 4, BSS1 and BSS2 constitute an ESS. In the WLANsystem, an STA is a device operating according to MAC/PHY regulation ofIEEE 802.11. STAs include AP STAs and non-AP STAs. The non-AP STAscorrespond to devices, such as mobile phones, handled directly by users.In FIG. 4, STA1, STA3, and STA4 correspond to the non-AP STAs and STA2and STA5 correspond to AP STAs.

In the following description, the non-AP STA may be referred to as aterminal, a wireless transmit/receive unit (WTRU), a user equipment(UE), a mobile station (MS), a mobile terminal, or a mobile subscriberstation (MSS). The AP is a concept corresponding to a base station (BS),a Node-B, an evolved Node-B (eNB), a base transceiver system (BTS), or afemto BS in other wireless communication fields.

Link Setup Process

FIG. 5 is a diagram for explaining a general link setup process.

In order to allow an STA to establish link setup on a network andtransmit/receive data over the network, the STA should perform processesof network discovery, authentication, association establishment,security setup, etc. The link setup process may also be referred to as asession initiation processor or a session setup process. In addition,discovery, authentication, association, and security setup of the linksetup process may also called an association process.

An exemplary link setup process is described with reference to FIG. 5.

In step S510, an STA may perform a network discovery action. The networkdiscovery action may include an STA scanning action. That is, in orderto access the network, the STA should search for an available network.The STA needs to identify a compatible network before participating in awireless network and the process of identifying the network present in aspecific area is referred to as scanning.

Scanning is categorized into active scanning and passive scanning.

FIG. 5 exemplarily illustrates a network discovery action including anactive scanning process. An STA performing active scanning transmits aprobe request frame in order to determine which AP is present in aperipheral region while moving between channels and waits for a responseto the probe request frame. A responder transmits a probe response framein response to the probe request frame to the STA that has transmittedthe probe request frame. Here, the responder may be an STA that hasfinally transmitted a beacon frame in a BSS of the scanned channel.Since an AP transmits a beacon frame in a BSS, the AP is a responder. Inan IBSS, since STAs of the IBSS sequentially transmit the beacon frame,a responder is not the same. For example, an STA, that has transmittedthe probe request frame at channel #1 and has received the proberesponse frame at channel #1, stores BSS-related information containedin the received probe response frame, and moves to the next channel(e.g. channel #2). In the same manner, the STA may perform scanning(i.e. probe request/response transmission and reception at Channel #2).

Although not shown in FIG. 5, the scanning action may also be carriedout using passive scanning. An STA that performs passive scanning awaitsreception of a beacon frame while moving from one channel to anotherchannel. The beacon frame is one of management frames in IEEE 802.11.The beacon frame is periodically transmitted to indicate the presence ofa wireless network and allow a scanning STA to search for the wirelessnetwork and thus join the wireless network. In a BSS, an AP isconfigured to periodically transmit the beacon frame and, in an IBSS,STAs in the IBSS are configured to sequentially transmit the beaconframe. Upon receipt of the beacon frame, the scanning STA storesBSS-related information contained in the beacon frame and records beaconframe information on each channel while moving to another channel. Uponreceiving the beacon frame, the STA may store BSS-related informationcontained in the received beacon frame, move to the next channel, andperform scanning on the next channel using the same method.

Active scanning is more advantageous than passive scanning in terms ofdelay and power consumption.

After discovering the network, the STA may perform an authenticationprocess in step S520. The authentication process may be referred to as afirst authentication process in order to clearly distinguish thisprocess from the security setup process of step S540.

The authentication process includes a process in which an STA transmitsan authentication request frame to an AP and the AP transmits anauthentication response frame to the STA in response to theauthentication request frame. The authentication frame used forauthentication request/response corresponds to a management frame.

The authentication frame may include information about an authenticationalgorithm number, an authentication transaction sequence number, a statecode, a challenge text, a robust security network (RSN), a finite cyclicgroup (FCG), etc. The above-mentioned information contained in theauthentication frame may correspond to some parts of information capableof being contained in the authentication request/response frame and maybe replaced with other information or include additional information.

The STA may transmit the authentication request frame to the AP. The APmay determine whether to permit authentication for the corresponding STAbased on the information contained in the received authenticationrequest frame. The AP may provide an authentication processing result tothe STA through the authentication response frame.

After the STA has been successfully authenticated, an associationprocess may be carried out in step S530. The association processincludes a process in which the STA transmits an association requestframe to the AP and the AP transmits an association response frame tothe STA in response to the association request frame.

For example, the association request frame may include informationassociated with various capabilities, a beacon listen interval, aservice set identifier (SSID), supported rates, supported channels, anRSN, a mobility domain, supported operating classes, a trafficindication map (TIM) broadcast request, interworking service capability,etc.

For example, the association response frame may include informationassociated with various capabilities, a status code, an association ID(AID), supported rates, an enhanced distributed channel access (EDCA)parameter set, a received channel power indicator (RCPI), a receivedsignal to noise indicator (RSNI), a mobility domain, a timeout interval(association comeback time), an overlapping BSS scan parameter, a TIMbroadcast response, a quality of service (QoS) map, etc.

The above-mentioned information may correspond to some parts ofinformation capable of being contained in the associationrequest/response frame and may be replaced with other information orinclude additional information.

After the STA has been successfully associated with the network, asecurity setup process may be performed in step S540. The security setupprocess of step S540 may be referred to as an authentication processbased on robust security network association (RSNA) request/response.The authentication process of step S520 may be referred to as a firstauthentication process and the security setup process of step S540 mayalso be simply referred to as an authentication process.

The security setup process of step S540 may include a private key setupprocess through 4-way handshaking based on, for example, an extensibleauthentication protocol over LAN (EAPOL) frame. In addition, thesecurity setup process may also be performed according to other securityschemes not defined in IEEE 802.11 standards.

WLAN Evolution

To overcome limitations of communication speed in a WLAN, IEEE 802.11nhas recently been established as a communication standard. IEEE 802.11naims to increase network speed and reliability and extend wirelessnetwork coverage. More specifically, IEEE 802.11n supports a highthroughput (HT) of 540 Mbps or more. To minimize transmission errors andoptimize data rate, IEEE 802.11n is based on MIMO using a plurality ofantennas at each of a transmitter and a receiver.

With widespread supply of a WLAN and diversified applications using theWLAN, the necessity of a new WLAN system for supporting a higherprocessing rate than a data processing rate supported by IEEE 802.11nhas recently emerged. A next-generation WLAN system supporting very highthroughput (VHT) is one of IEEE 802.11 WLAN systems which have beenrecently proposed to support a data processing rate of 1 Gbps or more ina MAC service access point (SAP), as the next version (e.g. IEEE802.11ac) of an IEEE 802.11n WLAN system.

To efficiently utilize a radio frequency (RF) channel, thenext-generation WLAN system supports a multiuser (MU)-MIMO transmissionscheme in which a plurality of STAs simultaneously accesses a channel.In accordance with the MU-MIMO transmission scheme, an AP maysimultaneously transmit packets to at least one MIMO-paired STA.

In addition, support of WLAN system operations in whitespace (WS) hasbeen discussed. For example, technology for introducing the WLAN systemin TV WS such as an idle frequency band (e.g. 54 to 698 MHz band) due totransition to digital TVs from analog TVs has been discussed under theIEEE 802.11af standard. However, this is for illustrative purposes only,and the WS may be a licensed band capable of being primarily used onlyby a licensed user. The licensed user is a user who has authority to usethe licensed band and may also be referred to as a licensed device, aprimary user, an incumbent user, etc.

For example, an AP and/or STA operating in WS should provide a functionfor protecting the licensed user. As an example, assuming that thelicensed user such as a microphone has already used a specific WSchannel which is a frequency band divided by regulations so as toinclude a specific bandwidth in the WS band, the AP and/or STA cannotuse the frequency band corresponding to the corresponding WS channel inorder to protect the licensed user. In addition, the AP and/or STAshould stop using the corresponding frequency band under the conditionthat the licensed user uses a frequency band used for transmissionand/or reception of a current frame.

Therefore, the AP and/or STA needs to determine whether a specificfrequency band of a WS band can be used, in other words, whether alicensed user is present in the frequency band. A scheme for determiningwhether a licensed user is present in a specific frequency band isreferred to as spectrum sensing. An energy detection scheme, a signaturedetection scheme, etc. are used as the spectrum sensing mechanism. TheAP and/or STA may determine that the frequency band is being used by alicensed user if the intensity of a received signal exceeds apredetermined value or if a DTV preamble is detected.

Machine-to-machine (M2M) communication technology has been discussed asnext generation communication technology. Technical standard forsupporting M2M communication has been developed as IEEE 802.11ah in anIEEE 802.11 WLAN system. M2M communication refers to a communicationscheme including one or more machines or may also be called machine typecommunication (MTC) or machine-to-machine communication. In this case,the machine refers to an entity that does not require directmanipulation or intervention of a user. For example, not only a meter orvending machine including a radio communication module but also a userequipment (UE) such as a smartphone capable of performing communicationby automatically accessing a network without usermanipulation/intervention may be machines. M2M communication may includedevice-to-device (D2D) communication and communication between a deviceand an application server. As exemplary communication between a deviceand an application server, communication between a vending machine andan application server, communication between a point of sale (POS)device and an application server, and communication between an electricmeter, a gas meter, or a water meter and an application server. M2Mcommunication-based applications may include security, transportation,healthcare, etc. In the case of considering the above-mentionedapplication examples, M2M communication has to support occasionaltransmission/reception of a small amount of data at low speed under anenvironment including a large number of devices.

More specifically, M2M communication should support a large number ofSTAs. Although a currently defined WLAN system assumes that one AP isassociated with a maximum of 2007 STAs, methods for supporting othercases in which more STAs (e.g. about 6000 STAs) than 2007 STAs areassociated with one AP have been discussed in M2M communication. Inaddition, it is expected that many applications forsupporting/requesting a low transfer rate are present in M2Mcommunication. In order to smoothly support these requirements, an STAin the WLAN system may recognize the presence or absence of data to betransmitted thereto based on a TIM element and methods for reducing thebitmap size of the TIM have been discussed. In addition, it is expectedthat much traffic having a very long transmission/reception interval ispresent in M2M communication. For example, a very small amount of datasuch as electric/gas/water metering needs to be transmitted and receivedat long intervals (e.g. every month). Accordingly, although the numberof STAs associated with one AP increases in the WLAN system, methods forefficiently supporting the case in which there are a very small numberof STAs each including a data frame to be received from the AP duringone beacon period has been discussed.

As described above, WLAN technology is rapidly developing and not onlythe above-mentioned exemplary technologies but also other technologiesincluding direct link setup, improvement of media streaming throughput,support of high-speed and/or large-scale initial session setup, andsupport of extended bandwidth and operating frequency are beingdeveloped.

Medium Access Mechanism

In a WLAN system based on IEEE 802.11, a basic access mechanism ofmedium access control (MAC) is a carrier sense multiple access withcollision avoidance (CSMA/CA) mechanism. The CSMA/CA mechanism is alsoreferred to as a distributed coordination function (DCF) of the IEEE802.11 MAC and basically adopts a “listen before talk” access mechanism.In this type of access mechanism, an AP and/or an STA may sense awireless channel or a medium during a predetermined time duration (e.g.DCF interframe space (DIFS) before starting transmission. As a result ofsensing, if it is determined that the medium is in an idle status, theAP and/or the STA starts frame transmission using the medium. Meanwhile,if it is sensed that the medium is in an occupied state, the AP and/orthe STA does not start its transmission and may attempt to perform frametransmission after setting and waiting for a delay duration (e.g. arandom backoff period) for medium access. Since it is expected thatmultiple STAs attempt to perform frame transmission after waiting fordifferent time durations by applying the random backoff period,collision can be minimized.

An IEEE 802.11 MAC protocol provides a hybrid coordination function(HCF) based on the DCF and a point coordination function (PCF). The PCFrefers to a scheme of performing periodic polling by using apolling-based synchronous access method so that all reception APs and/orSTAs can receive a data frame. The HCF includes enhanced distributedchannel access (EDCA) and HCF controlled channel access (HCCA). EDCA isa contention based access scheme used by a provider to provide a dataframe to a plurality of users. HCCA uses a contention-free based channelaccess scheme employing a polling mechanism. The HCF includes a mediumaccess mechanism for improving QoS of a WLAN and QoS data may betransmitted in both a contention period (CP) and a contention-freeperiod (CFP).

FIG. 6 is a diagram for explaining a backoff process.

Operations based on a random backoff period will now be described withreference to FIG. 6. If a medium of an occupy or busy state transitionsto an idle state, several STAs may attempt to transmit data (or frames).As a method for minimizing collision, each STA may select a randombackoff count, wait for a slot time corresponding to the selectedbackoff count, and then attempt to start data or frame transmission. Therandom backoff count may be a pseudo-random integer and may be set toone of 0 to CW values. In this case, CW is a contention window parametervalue. Although CWmin is given as an initial value of the CW parameter,the initial value may be doubled in case of transmission failure (e.g.in the case in which ACK for the transmission frame is not received). Ifthe CW parameter value reaches CWmax, the STAs may attempt to performdata transmission while CWmax is maintained until data transmission issuccessful. If data has been successfully transmitted, the CW parametervalue is reset to CWmin. Desirably, CW, CWmin, and CWmax are set to2^(n)−1 (where n=0, 1, 2, . . . ).

If the random backoff process is started, the STA continuously monitorsthe medium while counting down the backoff slot in response to thedetermined backoff count value. If the medium is monitored as theoccupied state, the countdown stops and waits for a predetermined time.If the medium is in the idle status, the remaining countdown restarts.

As shown in the example of FIG. 6, if a packet to be transmitted to MACof STA3 arrives at STA3, STA3 may confirm that the medium is in the idlestate during a DIFS and directly start frame transmission. In themeantime, the remaining STAs monitor whether the medium is in the busystate and wait for a predetermined time. During the predetermined time,data to be transmitted may occur in each of STA1, STA2, and STA5. If itis monitored that the medium is in the idle state, each STA waits forthe DIFS time and then may perform countdown of the backoff slot inresponse to a random backoff count value selected by each STA. Theexample of FIG. 6 shows that STA2 selects the lowest backoff count valueand STA1 selects the highest backoff count value. That is, after STA2finishes backoff counting, the residual backoff time of STA5 at a frametransmission start time is shorter than the residual backoff time ofSTA1. Each of STA1 and STA5 temporarily stops countdown while STA2occupies the medium, and waits for a predetermined time. If occupationof STA2 is finished and the medium re-enters the idle state, each ofSTA1 and STA5 waits for a predetermined time DIFS and restarts backoffcounting. That is, after counting down the remaining backoff timecorresponding to the residual backoff time, each of STA1 and STA5 maystart frame transmission. Since the residual backoff time of STA5 isshorter than that of STA1, STA5 starts frame transmission. Meanwhile,data to be transmitted may occur even in STA4 while STA2 occupies themedium. In this case, if the medium is in the idle state, STA4 may waitfor the DIFS time, perform countdown in response to the random backoffcount value selected thereby, and then start frame transmission. FIG. 6exemplarily shows the case in which the residual backoff time of STA5 isidentical to the random backoff count value of STA4 by chance. In thiscase, collision may occur between STA4 and STA5. Then, each of STA4 andSTA5 does not receive ACK, resulting in occurrence of data transmissionfailure. In this case, each of STA4 and STA5 may increase the CW valueby two times, select a random backoff count value, and then performcountdown. Meanwhile, STA1 waits for a predetermined time while themedium is in the occupied state due to transmission of STA4 and STA5. Ifthe medium is in the idle state, STA1 may wait for the DIFS time andthen start frame transmission after lapse of the residual backoff time.

STA Sensing Operation

As described above, the CSMA/CA mechanism includes not only a physicalcarrier sensing mechanism in which the AP and/or an STA directly sensesa medium but also a virtual carrier sensing mechanism. The virtualcarrier sensing mechanism can solve some problems such as a hidden nodeproblem encountered in medium access. For virtual carrier sensing, MACof the WLAN system may use a network allocation vector (NAV). The NAV isa value used to indicate a time remaining until an AP and/or an STAwhich is currently using the medium or has authority to use the mediumenters an available state to another AP and/or STA. Accordingly, a valueset to the NAV corresponds to a reserved time in which the medium willbe used by an AP and/or STA configured to transmit a correspondingframe. An STA receiving the NAV value is not allowed to perform mediumaccess during the corresponding reserved time. For example, NAV may beset according to the value of a ‘duration’ field of a MAC header of aframe.

A robust collision detection mechanism has been proposed to reduce theprobability of collision. This will be described with reference to FIGS.7 and 8. Although an actual carrier sensing range is different from atransmission range, it is assumed that the actual carrier sensing rangeis identical to the transmission range for convenience of description.

FIG. 7 is a diagram for explaining a hidden node and an exposed node.

FIG. 7(a) exemplarily shows a hidden node. In FIG. 7(a), STA Acommunicates with STA B, and STA C has information to be transmitted.Specifically, STA C may determine that a medium is in an idle state whenperforming carrier sensing before transmitting data to STA B, althoughSTA A is transmitting information to STA B. This is because transmissionof STA A (i.e. occupation of the medium) may not be detected at thelocation of STA C. In this case, STA B simultaneously receivesinformation of STA A and information of STA C, resulting in occurrenceof collision. Here, STA A may be considered a hidden node of STA C.

FIG. 7(b) exemplarily shows an exposed node. In FIG. 7(b), in asituation in which STA B transmits data to STA A, STA C has informationto be transmitted to STA D. If STA C performs carrier sensing, it isdetermined that a medium is occupied due to transmission of STA B.Therefore, although STA C has information to be transmitted to STA D,since the medium-occupied state is sensed, STA C should wait for apredetermined time until the medium is in the idle state. However, sinceSTA A is actually located out of the transmission range of STA C,transmission from STA C may not collide with transmission from STA Bfrom the viewpoint of STA A, so that STA C unnecessarily enters astandby state until STA B stops transmission. Here, STA C is referred toas an exposed node of STA B.

FIG. 8 is a diagram for explaining request to send (RTS) and clear tosend (CTS).

To efficiently utilize a collision avoidance mechanism under theabove-mentioned situation of FIG. 7, it is possible to use a shortsignaling packet such as RTS and CTS. RTS/CTS between two STAs may beoverheard by peripheral STA(s), so that the peripheral STA(s) mayconsider whether information is transmitted between the two STAs. Forexample, if an STA to be used for data transmission transmits an RTSframe to an STA receiving data, the STA receiving data may informperipheral STAs that itself will receive data by transmitting a CTSframe to the peripheral STAs.

FIG. 8(a) exemplarily shows a method for solving problems of a hiddennode. In FIG. 8(a), it is assumed that both STA A and STA C are ready totransmit data to STA B. If STA A transmits RTS to STA B, STA B transmitsCTS to each of STA A and STA C located in the vicinity of the STA B. Asa result, STA C waits for a predetermined time until STA A and STA Bstop data transmission, thereby avoiding collision.

FIG. 8(b) exemplarily shows a method for solving problems of an exposednode. STA C performs overhearing of RTS/CTS transmission between STA Aand STA B, so that STA C may determine that no collision will occuralthough STA C transmits data to another STA (e.g. STA D). That is, STAB transmits RTS to all peripheral STAs and only STA A having data to beactually transmitted may transmit CTS. STA C receives only the RTS anddoes not receive the CTS of STA A, so that it can be recognized that STAA is located outside of the carrier sensing range of STA C.

Power Management

As described above, the WLAN system needs to perform channel sensingbefore an STA performs data transmission/reception. The operation ofalways sensing the channel causes persistent power consumption of theSTA. Power consumption in a reception state is not greatly differentfrom that in a transmission state. Continuous maintenance of thereception state may cause large load to a power-limited STA (i.e. an STAoperated by a battery). Therefore, if an STA maintains a receptionstandby mode so as to persistently sense a channel, power isinefficiently consumed without special advantages in terms of WLANthroughput. In order to solve the above-mentioned problem, the WLANsystem supports a power management (PM) mode of the STA.

The PM mode of the STA is classified into an active mode and a powersave (PS) mode. The STA basically operates in the active mode. The STAoperating in the active mode maintains an awake state. In the awakestate, the STA may perform a normal operation such as frametransmission/reception or channel scanning. On the other hand, the STAoperating in the PS mode is configured to switch between a sleep stateand an awake state. In the sleep state, the STA operates with minimumpower and performs neither frame transmission/reception nor channelscanning.

Since power consumption is reduced in proportion to a specific time inwhich the STA stays in the sleep state, an operation time of the STA isincreased. However, it is impossible to transmit or receive a frame inthe sleep state so that the STA cannot always operate for a long periodof time. If there is a frame to be transmitted to an AP, the STAoperating in the sleep state is switched to the awake state totransmit/receive the frame. On the other hand, if the AP has a frame tobe transmitted to the STA, the sleep-state STA is unable to receive theframe and cannot recognize the presence of a frame to be received.Accordingly, the STA may need to switch to the awake state according toa specific period in order to recognize the presence or absence of aframe to be transmitted thereto (or in order to receive the frame if theAP has the frame to be transmitted thereto).

FIG. 9 is a diagram for explaining a PM operation.

Referring to FIG. 9, an AP 210 transmits a beacon frame to STAs presentin a BSS at intervals of a predetermined time period (S211, S212, S213,S214, S215, and S216). The beacon frame includes a TIM informationelement. The TIM information element includes buffered traffic regardingSTAs associated with the AP 210 and includes information indicating thata frame is to be transmitted. The TIM information element includes a TIMfor indicating a unicast frame and a delivery traffic indication map(DTIM) for indicating a multicast or broadcast frame.

The AP 210 may transmit a DTIM once whenever the beacon frame istransmitted three times. Each of STA1 220 and STA2 222 operate in a PSmode. Each of STA1 220 and STA2 222 is switched from a sleep state to anawake state every wakeup interval of a predetermined period such thatSTA1 220 and STA2 222 may be configured to receive the TIM informationelement transmitted by the AP 210. Each STA may calculate a switchingstart time at which each STA may start switching to the awake statebased on its own local clock. In FIG. 9, it is assumed that a clock ofthe STA is identical to a clock of the AP.

For example, the predetermined wakeup interval may be configured in sucha manner that STA1 220 can switch to the awake state to receive the TIMelement every beacon interval. Accordingly, STA1 220 may switch to theawake state when the AP 210 first transmits the beacon frame (S211).STA1 220 may receive the beacon frame and obtain the TIM informationelement. If the obtained TIM element indicates the presence of a frameto be transmitted to STA1 220, STA1 220 may transmit a power save-Poll(PS-Poll) frame, which requests the AP 210 to transmit the frame, to theAP 210 (S221 a). The AP 210 may transmit the frame to STA1 220 inresponse to the PS-Poll frame (S231). STA1 220 which has received theframe is re-switched to the sleep state and operates in the sleep state.

When the AP 210 secondly transmits the beacon frame, since a busy mediumstate in which the medium is accessed by another device is obtained, theAP 210 may not transmit the beacon frame at an accurate beacon intervaland may transmit the beacon frame at a delayed time (S212). In thiscase, although STA1 220 is switched to the awake state in response tothe beacon interval, STA1 does not receive the delay-transmitted beaconframe so that it re-enters the sleep state (S222).

When the AP 210 thirdly transmits the beacon frame, the correspondingbeacon frame may include a TIM element configured as a DTIM. However,since the busy medium state is given, the AP 210 transmits the beaconframe at a delayed time (S213). STA1 220 is switched to the awake statein response to the beacon interval and may obtain a DTIM through thebeacon frame transmitted by the AP 210. It is assumed that the DTIMobtained by STA1 220 does not have a frame to be transmitted to STA1 220and there is a frame for another STA. In this case, STA1 220 may confirmthe absence of a frame to be received in the STA1 220 and re-enters thesleep state so that the STA1 220 may operate in the sleep state. Aftertransmitting the beacon frame, the AP 210 transmits the frame to thecorresponding STA (S232).

The AP 210 fourthly transmits the beacon frame (S214). However, since itwas impossible for STA1 220 to obtain information regarding the presenceof buffered traffic associated therewith through previous doublereception of a TIM element, STA1 220 may adjust the wakeup interval forreceiving the TIM element. Alternatively, provided that signalinginformation for coordination of the wakeup interval value of STA1 220 iscontained in the beacon frame transmitted by the AP 210, the wakeupinterval value of the STA1 220 may be adjusted. In this example, STA1220, which has been switched to receive a TIM element every beaconinterval, may be configured to be switched to another operation state inwhich STA1 220 awakes from the sleep state once every three beaconintervals. Therefore, when the AP 210 transmits a fourth beacon frame(S214) and transmits a fifth beacon frame (S215), STA1 220 maintains thesleep state such that it cannot obtain the corresponding TIM element.

When the AP 210 sixthly transmits the beacon frame (S216), STA1 220 isswitched to the awake state and operates in the awake state, so that theSTA1 220 may obtain the TIM element contained in the beacon frame(S224). The TIM element is a DTIM indicating the presence of a broadcastframe. Accordingly, STA1 220 does not transmit the PS-Poll frame to theAP 210 and may receive the broadcast frame transmitted by the AP 210(S234). In the meantime, the wakeup interval configured for STA2 230 maybe longer than the wakeup interval of STA1 220. Accordingly, STA2 230may enter the awake state at a specific time (S215) where the AP 210fifthly transmits the beacon frame and receives the TIM element (S241).STA2 230 may recognize the presence of a frame to be transmitted theretothrough the TIM element and transmit the PS-Poll frame to the AP 210 torequest frame transmission (S241 a). The AP 210 may transmit the frameto STA2 230 in response to the PS-Poll frame (S233).

In order to manage a PS mode shown in FIG. 9, the TIM element mayinclude either a TIM indicating the presence or absence of a frame to betransmitted to the STA or include a DTIM indicating the presence orabsence of a broadcast/multicast frame. The DTIM may be implementedthrough field setting of the TIM element.

FIGS. 10 to 12 are diagrams for explaining detailed operations of an STAthat has received a TIM.

Referring to FIG. 10, an STA is switched from a sleep state to an awakestate so as to receive a beacon frame including a TIM from an AP. TheSTA may recognize the presence of buffered traffic to be transmittedthereto by interpreting the received TIM element. After contending withother STAs to access a medium for PS-Poll frame transmission, the STAmay transmit the PS-Poll frame for requesting data frame transmission tothe AP. Upon receiving the PS-Poll frame transmitted by the STA, the APmay transmit the frame to the STA. The STA may receive a data frame andthen transmit an ACK frame to the AP in response to the received dataframe. Thereafter, the STA may re-enter the sleep state.

As illustrated in FIG. 10, the AP may operate according to an immediateresponse scheme in which the AP receives the PS-Poll frame from the STAand transmits the data frame after a predetermined time (e.g. a shortinterframe space (SIFS)). Meanwhile, if the AP does not prepare a dataframe to be transmitted to the STA during the SIFS time after receivingthe PS-Poll frame, the AP may operate according to a deferred responsescheme and this will be described with reference to FIG. 11.

The STA operations of FIG. 11 in which an STA is switched from a sleepstate to an awake state, receives a TIM from an AP, and transmits aPS-Poll frame to the AP through contention are identical to those ofFIG. 10. Even upon receiving the PS-Poll frame, if the AP does notprepare a data frame during an SIFS time, the AP may transmit an ACKframe to the STA instead of transmitting the data frame. If the dataframe is prepared after transmission of the ACK frame, the AP maytransmit the data frame to the STA after completion of contention. TheSTA may transmit the ACK frame indicating that the data frame hassuccessfully been received to the AP and transition to the sleep state.

FIG. 12 illustrates an exemplary case in which an AP transmits a DTIM.STAs may be switched from the sleep state to the awake state so as toreceive a beacon frame including a DTIM element from the AP. The STAsmay recognize that a multicast/broadcast frame will be transmittedthrough the received DTIM. After transmission of the beacon frameincluding the DTIM, the AP may directly transmit data (i.e. themulticast/broadcast frame) without transmitting/receiving a PS-Pollframe. While the STAs continuously maintains the awake state afterreception of the beacon frame including the DTIM, the STAs may receivedata and then switch to the sleep state after completion of datareception.

TIM Structure

In the operation and management method of the PS mode based on the TIM(or DTIM) protocol described with reference to FIGS. 9 to 12, STAs maydetermine whether a data frame to be transmitted for the STAs throughSTA identification information contained in a TIM element. The STAidentification information may be information associated with an AID tobe allocated when an STA is associated with an AP.

The AID is used as a unique ID of each STA within one BSS. For example,the AID for use in the current WLAN system may be allocated as one of 1to 2007. In the currently defined WLAN system, 14 bits for the AID maybe allocated to a frame transmitted by an AP and/or an STA. Although theAID value may be assigned up to 16383, the values of 2008 to 16383 areset to reserved values.

A TIM element according to legacy definition is inappropriate to applyan M2M application through which many STAs (for example, more than 2007STAs) are associated with one AP. If a conventional TIM structure isextended without any change, since the TIM bitmap size excessivelyincreases, it is impossible to support the extended TIM structure usinga legacy frame format and the extended TIM structure is inappropriatefor M2M communication in which application of a low transfer rate isconsidered. In addition, it is expected that there are a very smallnumber of STAs each having a reception data frame during one beaconperiod. Therefore, according to exemplary application of theabove-mentioned M2M communication, since it is expected that most bitsare set to zero (0) although the TIM bitmap size is increased,technology capable of efficiently compressing a bitmap is needed.

In legacy bitmap compression technology, successive values of 0 areomitted from a front part of a bitmap and the omitted result may bedefined as an offset (or start point) value. However, although STAs eachincluding a buffered frame is small in number, if there is a highdifference between AID values of respective STAs, compression efficiencyis not high. For example, assuming that only a frame to be transmittedto two STAs having AID values of 10 and 2000 is buffered, the length ofa compressed bitmap is set to 1990 but the remaining parts other thanboth end parts are assigned zero. If fewer STAs are associated with oneAP, inefficiency of bitmap compression does not cause serious problems.However, if the number of STAs associated with one AP increases, suchinefficiency may deteriorate overall system performance.

In order to solve the above-mentioned problems, AIDs are divided into aplurality of groups such that data can be more efficiently transmitted.A designated group ID (GID) is allocated to each group. AIDs allocatedon a group basis will be described with reference to FIG. 13.

FIG. 13(a) is a diagram illustrating an exemplary group-based AID. InFIG. 13(a), a few bits located at the front part of an AID bitmap may beused to indicate a GID. For example, it is possible to designate fourGIDs using the first two bits of an AID bitmap. If a total length of theAID bitmap is N bits, the first two bits (B1 and B2) may represent a GIDof the corresponding AID.

FIG. 13(a) is a diagram illustrating another exemplary group-based AID.In FIG. 13(b), a GID may be allocated according to the position of theAID. In this case, AIDs having the same GID may be represented by offsetand length values. For example, if GID 1 is denoted by offset A andlength B, this means that AIDs of A to A+B−1 on a bitmap have GID 1. Forexample, FIG. 13(b) assumes that AIDs of 1 to N4 are divided into fourgroups. In this case, AIDs contained in GID 1 are denoted by 1 to N1 andthe AIDs contained in this group may be represented by offset 1 andlength N1. Next, AIDs contained in GID 2 may be represented by offsetN1+1 and length N2−N1+1, AIDs contained in GID 3 may be represented byoffset N2+1 and length N3−N2+1, and AIDs contained in GID 4 may berepresented by offset N3+1 and length N4−N3+1.

If the aforementioned group-based AIDs are introduced, channel accessmay be allowed in a different time interval according to GIDs, so thatthe problem caused by the insufficient number of TIM elements withrespect to a large number of STAs can be solved and at the same timedata can be efficiently transmitted/received. For example, during aspecific time interval, channel access is allowed only for STA(s)corresponding to a specific group and channel access to the remainingSTA(s) may be restricted. A predetermined time interval in which accessto only specific STA(s) is allowed may also be referred to as arestricted access window (RAW).

Channel access based on GID will now be described with reference to FIG.13(c). FIG. 13(c) exemplarily illustrates a channel access mechanismaccording to a beacon interval when AIDs are divided into three groups.A first beacon interval (or a first RAW) is a specific interval in whichchannel access to STAs corresponding to AIDs contained in GID 1 isallowed and channel access of STAs contained in other GIDs isdisallowed. To implement this, a TIM element used only for AIDscorresponding to GID 1 is contained in a first beacon. A TIM elementused only for AIDs corresponding to GID 2 is contained in a secondbeacon frame. Accordingly, only channel access to STAs corresponding tothe AIDs contained in GID 2 is allowed during a second beacon interval(or a second RAW). A TIM element used only for AIDs having GID 3 iscontained in a third beacon frame, so that channel access to STAscorresponding to the AIDs contained in GID 3 is allowed during a thirdbeacon interval (or a third RAW). A TIM element used only for AIDshaving GID 1 is contained in a fourth beacon frame, so that channelaccess to STAs corresponding to the AIDs contained in GID 1 is allowedduring a fourth beacon interval (or a fourth RAW). Thereafter, onlychannel access to STAs belonging to a specific group indicated by a TIMcontained in a corresponding beacon frame may be allowed in each ofbeacon intervals subsequent to the fifth beacon interval (or in each ofRAWs subsequent to the fifth RAW).

Although FIG. 13(c) exemplarily shows that the order of allowed GIDs iscyclical or periodic according to the beacon interval, the scope of thepresent invention is not limited thereto. That is, only AID(s) containedin specific GID(s) may be contained in a TIM element, so that channelaccess only to STA(s) corresponding to the specific AID(s) is allowedduring a specific time interval (e.g. a specific RAW) and channel accessto the remaining STA(s) is disallowed.

The aforementioned group-based AID allocation scheme may also bereferred to as a hierarchical structure of a TIM. That is, a total AIDspace is divided into a plurality of blocks and channel access to STA(s)(i.e. STA(s) of a specific group) corresponding to a specific blockhaving any one of values other than ‘0’ may be allowed. Therefore, sincea large-sized TIM is divided into small-sized blocks/groups, an STA caneasily maintain TIM information and blocks/groups may be easily managedaccording to class, QoS or usage of the STA. Although FIG. 13exemplarily shows a 2-level layer, a hierarchical TIM structurecomprised of two or more levels may be configured. For example, a totalAID space may be divided into a plurality of page groups, each pagegroup may be divided into a plurality of blocks, and each block may bedivided into a plurality of sub-blocks. In this case, according to theextended version of FIG. 13(a), first N1 bits of an AID bitmap mayrepresent a page ID (i.e. PID), the next N2 bits may represent a blockID, the next N3 bits may represent a sub-block ID, and the remainingbits may represent the position of STA bits contained in a sub-block.

In the embodiments of the present invention described below, variousschemes for dividing STAs (or AIDs allocated to the STAs respectively)into predetermined hierarchical group units and managing the same may beused, but the group-based AID allocation schemes are not limited tothese embodiments.

Restricted Access Window (RAW)

Collision occurring between STAs that perform access simultaneously mayreduce medium utilization. As a method to distribute channel access from(group-based) STAs, a RAW may be used. An AP may assign a medium accessinterval called RAW between beacon intervals. RAW-related information (aRestricted Access Window Parameter Set (RPS) element) may be transmittedin a (short) beacon frame. In addition to the RAW, the AP may furtherassign one or more different RAWs related to other RAW parameters foranother group between the beacon intervals.

FIG. 14 shows an exemplary RAW. Referring to FIG. 14, STAs of a specificgroup corresponding to an RAW may perform access in the RAW (morespecifically, in one of the slots of the RAW). Herein, the specificgroup may be indicated by, for example, a RAW Group field, which will bedescribed later. In other words, an STA may recognize whether the AIDthereof corresponds to a specific group (RAW group) by determiningwhether or not the AID is within an AID range indicated by, for example,the RAW Group field. For example, if the AID of the STA is greater thanor equal to the lowest AID(N1) allocated to the RAW and less than orequal to the highest AID(N1) allocated to the RAW, the STA may beconsidered as belonging to a RAW group indicated by the RAW Group field.Herein, N1 may be determined by a concatenation of a Page Index subfieldand an RAW Start AID subfield, and N2 may be determined by aconcatenation of the Page Index subfield and an RAW End AID subfield.The subfields may be included in the RAW Group subfield in the RPSelement.

If the STA corresponds to the RAW group illustrated in FIG. 14 (and ispaged), the STA may perform access by transmitting a PS-Poll frame basedon the DCF and EDCA in the slot allocated thereto. Herein, the allocatedslot may be a slot allocated by the AP among the slots included in theRAW. The slot may be allocated in a manner as shown in FIG. 15. In FIGS.15(a) and 15(b), a slot is basically determined byi_(slot)=(x+N_(offset))mod N_(RAW), wherein x is the AID of the STA,i_(slot) is the slot index allocated to the STA, N_(offset) denotes twoleast significant bytes (LSBs) of an FCS field of the (short) beaconframe, and N_(RAW) is the number of time slots included in the RAW,which may be determined by a RAW Slot Definition subfield in the RPSelement. FIG. 15(a) illustrates allocation of slots to AIDs performedregardless of whether the AID is set to 1 in the TIM bitmap, and FIG.15(b) illustrates allocation of slots to only AIDs set to 1 in the TIMbitmap.

Restricted Access Window Parameter Set (RPS) Element

The RPS element includes a parameter set necessary for the RAW describedabove. This information field includes RAW Assignment fields for Groups1 to N. FIG. 16 shows an RPS element. Specifically, FIG. 16(a) showfields constituting the RPS element, FIG. 16(b) shows subfieldsconstituting the RAW N Assignment field, FIG. 16(c) shows configurationof a RAW Group subfield among the subfields of the RAW N Assignmentfield, and FIG. 16(d) shows configuration of an Options subfield amongthe subfields of the RAW N Assignment field.

Referring to FIG. 16(a), the RPS element may include an Element IDfield, a Length field, and a RAW N Assignment field.

Referring to FIG. 16(b), the RAW N Assignment field may include a PRAWIndication subfield, a Same Group Indication subfield, a RAW Groupsubfield, a RAW Start Time subfield, a RAW Duration subfield, an Optionssubfield, and a RAW Slot Definition subfield.

The PRAW Indication subfield indicates whether the current RAWAssignment field is a normal RAW or a PRAW. The Same Group Indicationsubfield indicates whether a RAW group related to the current RAWAssignment field is the same as the RAW group defined in the previousRAW Assignment field. If the Same Group Indication subfield is set to 1,this indicates that the RAW group of the current RAW Assignment field isthe same as the RAW group defined in the previous RAW Assignment field.In this case, the current RAW Assignment field does not include the RAWGroup field. The RAW Group subfield indicates the AID range of the STAsof the group related to the current RAW Assignment field. As shown inFIG. 16(c), the RAW Group field may include Page Index, RAW Start AIDand RAW End AID subfields. Since description of how the range of AID isdetermined by these subfields has been given above in relation to theRAW, it will not be given below.

The RAW Start Time subfield indicates time from the end time of beacontransmission to the start time of the RAW in units of TU. The RAWDuration subfield indicates the duration, in TU, of restricted mediumaccess which is allocated to the RAW. The Options subfield includes anAccess Restricted to Paged STAs Only subfield, which indicates whetheronly paged STAs are allowed to perform access in the RAW. The RAW SlotDefinition subfield may include a Slot Duration subfield, a SlotAssignments subfield, and a Cross Slot Boundary subfield. For details ofinformation which is included in the RPS element but is not describedabove and information/fields which are not specifically described above,refer to IEEE P802.11ah/D0.1.

As described above, the RAW Start Time subfield indicates time from theend time of beacon transmission to the start time of the RAW, therebyindicating the start time of a RAW related to the current RAW Assignmentfield. If the RAW starts immediately after a transmission interval of aspecific frame or a specific period, and thus the start time of the RAWcan be identified without the time from the end time of beacontransmission to the start time of the RAW being known, it may not beneeded to include the RAW Start Time subfield in every RAW Assignmentfield. If not all the RAW Assignment fields need to include the RAWStart Time subfield, the size of the beacon frame may be reduced, whichwill be significantly advantageous in terms of signaling overhead.Hereinafter, relevant embodiments of the present invention will bediscussed.

Embodiments

An STA may receive a beacon frame containing an RPS element from an APand check a RAW Assignment field in the RPS element. If the STAcorresponds to a RAW group related to the RAW Assignment field, the STAmay perform access based on the start time of the RAW related to the RAWAssignment field. As described above, channel access is performed in aslot allocated to the STA in the RAW. In order to identify the starttime of the slot allocated to the STA, the STA basically needs toidentify the start time of the RAW. Herein, the start time of the RAWmay be determined based on a Start Time Indication subfield (also calledRAW Start Time Present), which indicates whether or not the RAW StartTime subfield indicating the start time of the RAW is included in theRAW Assignment field.

More specifically, the Start Time Indication subfield indicates whetheror not the RAW Start Time subfield is included in the RAW Assignmentfield. If this subfield is set to 0, the RAW Assignment field may notinclude the RAW Start Time subfield. In this case, there is noinformation which directly indicates the start time of the RAW, and thusthe start time of the RAW may be determined as follows.

If the RAW Assignment field is the first RAW Assignment field of the RPSelement, the start time of the RAW may be the time immediately aftertransmission of the (short) beacon frame transmitting the RPS element.In other words, if the Start Time Indication subfield is set to 0 in thefirst RAW Assignment field, this may indicate that the RAW startsimmediately after the (short) beacon frame.

If the RAW Assignment field is the second RAW Assignment field or a RAWAssignment field after the second RAW assignment field in the RPSelement, the start time of the RAW may be the time immediately after endof the previous RAW. In other words, if the Start Time Indicationsubfield is set to 0 in a RAW Assignment field other than the first RAWAssignment field, this may indicate that the RAW starts immediatelyafter the previous RAW.

That is, if the value of the Start Time Indication subfield is 0, thestart time of the RAW may be considered as being determined depending onthe position of the RAW Assignment field in the RPS element.

Next, if the Start Time Indication subfield is set to 1, the start timeof the RAW may be determined by the RAW Start Time subfield, and the RAWAssignment field may include a RAW Start Time subfield.

FIG. 17 illustrates an example of the RAW Assignment field including theStart Time Indication subfield. Specifically, FIG. 17(a) illustrates aRAW Assignment field including a RAW Start Time Indication subfield, andFIG. 17(b) illustrates a RAW Assignment field including no RAW StartTime field when the Start Time Indication subfield is set to 0.Configuration of the RAW Assignment field and names of the subfieldsthereof in FIG. 17 are simply illustrative. It should be noted thatembodiments of the present invention are not limited to the RAWAssignment field illustrated in FIG. 17.

Meanwhile, whether or not the STA belongs to a RAW group related to thecurrent RAW Assignment field may be basically determined by the RAWgroup field. If the RAW group related to the RAW Assignment field is thesame as the group information indicated by the TIM, a field indicatingthis may eliminate a constraint that all RAW Assignment fields shouldinclude a RAW group field. Specifically, a Same TIM Indication subfieldto indicate whether or not the RAW has been assigned the sameinformation as the information on all STAs (group and AID information onSTAs) may be used. FIG. 18 illustrates a RAW Assignment field includingthe Same TIM Indication subfield.

If the Same TIM Indication subfield is set to 1, this means that groupinformation, which is the same as the information on all the STAsindicated by the TIM, is given. In this case, the AP may omit the RAWgroup subfield from the RPS element. If the Same TIM Indication subfieldis set to 0, this means that the group information in the TIM isunrelated to the current RAW. Accordingly, a separate RAW group subfieldis needed. FIG. 28 shows examples of a RAW Assignment field for theaforementioned two cases.

FIGS. 20 and 21 illustrate application of the RAW Assignment fieldaccording to a second embodiment. Referring to FIG. 20, since the SameTIM Indication subfield is set to 1, AIDs 1 to 8, which is the range ofAIDs indicated by the TIM bitmap, may correspond to the RAW group of theRAW Assignment field. In particular, FIG. 20 illustrates assignment ofthe RAW to only paged STAs in the TIM with the Options set to 1.

FIG. 21 illustrates application of the RAW Assignment field with a TIMbitmap for Page 1 and a TIM bitmap for Page 2. Since the Same TIMIndication subfields for the TIM bitmap are both set to 1, the AID rangeof a RAW group related to each RAW Assignment field is indicated by theTIM for each page.

Table 1 given below shows bits necessary for the conventional RAWAssignment field, and Table 2 shows bits which are needed for a RAWAssignment field when the Same TIM Indication subfield is applied.

TABLE 1 Feature Value (bits) IE type 8 IE length 8 PRAW Indication (0) 1Same Group Indication 1 Page ID 2 RAW Start AID 11 RAW End AID 11 RAWStart Time 8 RAW Duration 8 Access restriction 1 Frame Type Restriction1 Group/RA frame indication 1 RAW Slot definition 12 Channel 8 AP PM 1Reserved 6 Total: 88

TABLE 2 Feature Value (bits) IE type 8 IE length 8 PRAW Indication (0) 1Same TIM Indication 1 Same Group Indication 1 Page ID 2 RAW Start AID 11RAW End AID 11 RAW Start Time 8 RAW Duration 8 Access restriction 1Frame Type Restriction 1 Group/RA frame indication 1 RAW Slot definition12 Channel 8 AP PM 1 Reserved 5 Total: 88

According to Tables 1 and 2, if one TIM IE is transmitted (i.e., a TIMIE is transmitted for one page), and two RAWs are assigned, theconventional method needs IE Type & Length (16 bits)+2×RAW N Assignment(72 bits)=20 bytes (160 bits). On the other hand, in Embodiment 2, IEType & Length (16 bits)+2×RAW N Assignment (48 bits)=14 bytes (112 bits)are needed, and thus the number of bits may be reduced by 6 bytes(Gain=20% overhead reduction).

If two TIM IEs are transmitted (i.e., TIM IEs are transmitted for twopages), and two RAWs are assigned, a PS-Poll RAW and a data RAW areassigned to each page, the conventional method needs IE Type & Length(16 bits)+4×RAW N Assignment (72 bits)=38 bytes (304 bits). On the otherhand, in Embodiment 2, IE Type & Length (16 bits)+2×RAW N Assignment (48bits)=14 bytes (112 bits) are needed, and thus the number of bits may bereduced by 24 bytes (Gain=63% overhead reduction).

FIG. 22 illustrates use of both the Start Time Indication subfield andthe Same TIM Indication subfield.

Referring to FIG. 22, since the Start Time Indication subfield 2211 isset to 0 in the RAW 1 Assignment field 2210, the STA may recognize thatRAW 1 starts immediately after transmission of a beacon frame. The STAmay also recognize, based on the Same TIM Indication 2212 set to 1, thatthe RAW group related to the RAW 1 Assignment field 2210 is identical toa group (with the AID range of AIDs 1 to 8) indicated by the TIM bitmapfor Page 1.

Further, since the Start Time Indication subfield 2221 is set to 0 inthe RAW 2 Assignment field 2220, the STA may recognize that RAW 2 startsimmediately after transmission of RAW 1. The STA may also recognize,based on the Same TIM Indication 2222 set to 1, that the RAW grouprelated to the RAW 2 Assignment field 2220 is identical to a group (withthe AID range of AIDs 33 to 40) indicated by the TIM bitmap for Page 2.

Referring to FIG. 22, since the Start Time Indication subfields of theRAW 1 Assignment field 2210 and RAW 2 Assignment field 2220 are both setto 0, neither the RAW 1 Assignment field 2210 nor RAW 2 Assignment field2220 may include a RAW Start Time field. In addition, since the Same TIMIndication field is set to 1 in both RAW Assignment fields, none of theRAW Assignment fields may include a RAW group field.

Table 3 given below shows the number of bits necessary for a RAWAssignment field.

TABLE 3 Feature Value IE type 8 IE length 8 PRAW Indication (0) 1 SameTIM Indication 1 RAW Start Time Indication 1 Same Group Indication 1Page ID 2 RAW Start AID 11 RAW End AID 11 RAW Start Time 8 RAW Duration8 Access restriction 1 Frame Type Restriction 1 Group/RA frameindication 1 RAW Slot definition 12 Channel 8 AP PM 1 Reserved 4 Total:88

Specifically, if two TIM IEs are transmitted (i.e., TIM IEs aretransmitted for two page), and two RAWs are assigned, a PS-Poll RAW anda data RAW are assigned to each page, the conventional method needs IEType & Length (16 bits)+4×RAW N Assignment (72 bits)=38 bytes (304bits). If the Same TIM Indication subfield is set to 1 and the StartTime Indication subfield is set to 0, the number of necessary bits is IEType & Length (16 bits)+2×RAW N Assignment (40 bits)=13 bytes (112bits). That is, the bits may be reduced by 25 bytes compared to theconventional method (Gain=65%).

Details of various embodiments of the present invention described abovemay be independently employed or a combination of two or moreembodiments may be implemented.

Configuration of Apparatus According to Embodiment of the PresentInvention

FIG. 23 is a block diagram illustrating wireless apparatuses accordingto one embodiment of the present invention.

An AP 10 may include a processor 11, a memory 12, and a transceiver 13.An STA 20 may include a processor 21, a memory 22, and a transceiver 23.The transceivers 13 and 23 may transmit/receive wireless signals andimplement, for example, a physical layer according to an IEEE 802system. The processors 11 and 21 may be connected to the transceivers 13and 23 to implement a physical layer and/or MAC layer according to anIEEE 802 system. The processors 11 and 21 may be configured to performoperations according to the various embodiments of the present inventiondescribed above. In addition, modules to implement operations of the APand STA according to the various embodiments of the present inventiondescribed above may be stored in the memories 12 and 22 and executed bythe processors 11 and 21. The memories 12 and 22 may be contained in orinstalled outside the processors 11 and 21 and connected to theprocessors 11 and 21 via well-known means.

Configuration of the AP and the STA may be implemented such that thedetails of the various embodiments of the present invention describedabove are independently applied or a combination of two or moreembodiments is applied. For clarity, redundant description is omitted.

Embodiments of the present invention may be implemented by various meanssuch as, for example, hardware, firmware, software, or combinationsthereof.

When embodied as hardware, methods according to embodiments of thepresent invention may be implemented by one or more ASICs (applicationspecific integrated circuits), DSPs (digital signal processors), DSPDs(digital signal processing devices), PLDs (programmable logic devices),FPGAs (field programmable gate arrays), processors, controllers,microcontrollers, microprocessors, and the like.

When embodied as firmware or software, methods according to embodimentsof the present invention may be implemented in the form of a module, aprocedure, a function, or the like which performs the functions oroperations described above. Software code may be stored in the memoryunit and executed by the processor. The memory unit may be disposedinside or outside the processor to transceive data with the processorvia various well-known means.

Preferred embodiments of the present invention have been described indetail above to allow those skilled in the art to implement and practicethe present invention. Although the preferred embodiments of the presentinvention have been described above, those skilled in the art willappreciate that various modifications and variations can be made in thepresent invention without departing from the spirit and scope of theinvention set forth in the claims below. Thus, the present invention isnot intended to be limited to the embodiments described herein, but isintended to include the widest range of embodiments corresponding to theprinciples and novel features disclosed herein.

INDUSTRIAL APPLICABILITY

Various embodiments of the present invention have been described throughexamples applied to an IEEE 802.11 system, but they may also be appliedto other wireless access systems in the same manner.

What is claimed is:
 1. A method for performing access to a medium by astation, STA, in a wireless communication system, the method comprising:receiving a frame containing a Restricted Access Window, RAW, ParameterSet, RPS, element; checking a RAW Assignment field in the RPS element;and performing access based on a start time of a RAW when the STAcorresponds to a RAW group related to the RAW Assignment field, whereinthe start time of the RAW is obtained based on a Start Time Indicationsubfield, wherein the Start Time Indication subfield indicates whether aRAW Start Time subfield indicating the start time of the RAW is includedin the RAW Assignment field, wherein, when the RAW Assignment field is afirst RAW Assignment field in the RPS element and the Start TimeIndication subfield is set to 0, the start time of the RAW is a timeimmediately after transmission of the frame, and wherein, when the RAWAssignment field is not the first RAW Assignment field in the RPSelement and the Start Time Indication subfield is set to 0, the starttime of the RAW is determined depending on a position of the RAWAssignment field among other RAW Assignment fields in the RPS element.2. The method according to claim 1, wherein, when the RAW Assignmentfield is not the first RAW Assignment field in the RPS element and theStart Time Indication subfield is set to 0, the RAW start time is a timeimmediately after end of a previous RAW.
 3. The method according toclaim 1, wherein, when the Start Time Indication subfield is set to 0,the RAW Assignment field does not include the RAW Start Time subfield.4. The method according to claim 1, wherein, when the Start TimeIndication subfield is set to 1, the start time of the RAW is determinedby the RAW Start Time subfield.
 5. The method according to claim 1,wherein, when the Start Time Indication subfield is set to 1, the RAWAssignment field comprises the RAW Start Time subfield.
 6. The methodaccording to claim 1, wherein the RAW Start Time subfield indicates aduration from the frame to the start time of the RAW.
 7. The methodaccording to claim 1, wherein the frame is either a beacon frame or ashort beacon frame.
 8. The method according to claim 1, wherein theperforming access comprises: determining a slot for performing accessamong one or more slots included in the RAW; and performingcontention-based access in the determined slot.
 9. The method accordingto claim 8, wherein the slot for performing access is determined by anassociation identifier, AID, of the STA.
 10. The method according toclaim 1, wherein the RPS element contains one or more RAW Assignmentfields.
 11. A station for performing access to a medium in a wirelesscommunication system, the station comprising: a transceiver module; anda processor, wherein the processor is configured to: receive a framecontaining a Restricted Access Window, RAW, Parameter Set, RPS, element;check a RAW Assignment field in the RPS element; and perform accessbased on a start time of a RAW when the STA corresponds to a RAW grouprelated to the RAW Assignment field, wherein the start time of the RAWis obtained based on a Start Time Indication subfield, wherein the StartTime Indication subfield indicates whether a RAW Start Time subfieldindicating the start time of the RAW is included in the RAW Assignmentfield, wherein, when the RAW Assignment field is a first RAW Assignmentfield in the RPS element and the Start Time Indication subfield is setto 0, the start time of the RAW is a time immediately after transmissionof the frame, and wherein, when the RAW Assignment field is not thefirst RAW Assignment field in the RPS element and the Start TimeIndication subfield is set to 0, the start time of the RAW is determineddepending on a position of the RAW Assignment field among other RAWAssignment fields in the RPS element.
 12. The station according to claim11, wherein, when the RAW Assignment field is not the first RAWAssignment field in the RPS element and the Start Time Indicationsubfield is set to 0, the RAW start time is a time immediately after endof a previous RAW.
 13. The station according to claim 11, wherein, whenthe Start Time Indication subfield is set to 0, the RAW Assignment fielddoes not include the RAW Start Time subfield.
 14. The station accordingto claim 11, wherein, when the Start Time Indication subfield is set to1, the start time of the RAW is determined by the RAW Start Timesubfield.
 15. The station according to claim 11, wherein, when the StartTime Indication subfield is set to 1, the RAW Assignment field comprisesthe RAW Start Time subfield.
 16. The station according to claim 11,wherein the RAW Start Time subfield indicates a duration from the frameto the start time of the RAW.
 17. The station according to claim 11,wherein the processor performs access comprising: determining a slot forperforming access among one or more slots included in the RAW; andperforming contention-based access in the determined slot.